Micromachined structures have become increasingly important for a variety of applications. Movable mechanical elements such as cantilevers, beams, and diaphragms are often micromachined for sensors and actuators. Surface micromachining is a microfabrication technology that has attracted great attention, partly because it can produce micromachined structures that can be integrated with electronic devices. With this technology, a sacrificial layer, such as silicon oxide, is first deposited and patterned on a substrate, usually a silicon wafer already coated with silicon nitride. The film for the microstructure is then deposited and patterned. The sacrificial layer is then etched away to release the microstructure, leaving it freely suspended and anchored only where it directly contacts the substrate through the patterned opening of the sacrificial layer.
The large surface area to volume ratios of these microstructures, whether processed by surface micromachining, bulk micromachining, wafer dissolving and bonding process, or LIGA process, however, result in problems associated with unwanted adhesion between adjacent elements. Such adhesive failures have a direct impact on production yield and reliability of these devices.
The sticking of freely standing microstructures to the substrate is a principal source of failures in surface micromachined devices. Stiction occurs both immediately after the sacrificial etch release process and during operation of the device. During the rinse process and after release of the sacrificial layer by the wet etch process, the capillary force from the rinse liquid causes attraction between suspended elements of the device and the underlying substrate which causes these elements to adhere to the underlying substrate. Even after a successful release, problems with stiction may still arise if the microstructure is exposed to liquid, from any subsequent wet process, or water vapor condensation during device operation.
Previous approaches may reduce the likelihood of an adhesion failure to some degree but fail to eliminate the problem altogether. An example of a proposed approach is described in the Abe et al. reference, published in the Proc. IEEE Mems Workshop, p. 94, published on Jan. 29, 1995 in the Netherlands. This reference describes a process whereby the surface contact area is reduced by introducing bumps at the bottom of the surface of the freely standing structure. Another approach is published in Transducers '93, p. 288, by Alley et al., 1993 in Yokohama, Japan. The Alley et al. reference outlines different ways of increasing surface roughness of the substrate to reduce the real surface contact area. In another publication by Mulhern et al., Transducers '93, p. 296, 1993 in Yokohama, Japan, there is discussed a method of using super critical carbon dioxide drying to prevent stiction by eliminating capillary forces from the rinse liquid. However, this approach cannot eliminate post-rinse stiction problems. Yet another approach is presented by Houston et al. in Proc. of the 8th International Conference on Solid-State Sensors and Actuators, p. 210, Stockholm, Sweden, June 1995, wherein ammonium fluoride is used to treat a polysilicon structure surface to obtain a passivated hydrogen terminated surface. However, in the presence of air, the polysilicon surface oxidizes in less than one week and loses all the benefits from the treatment.
There thus remains a need for a method of reducing and preventing the stiction of micromachined structures for the life of a micromachined device.